2. You are the only mystery worth solving.
Every companion has a moment when they come face-to-face with the reality of the sliver of ice in the Doctors heart. The Snowmen Doctors response to the inevitability of death, of endings, was to withdraw from the world. But even then, how could Clara, how could all of us, not be ghosts to him? Why would the Doctor ever care about any of us? The Doctors response is that we are the only mystery worth solving. Once again, Smith delivers this line with an incongruous gentle grin. Its the best way the Doctor can think of to express why hes engaged with the world again, why he engages with people and their stories the way he does. What could we possibly be to the Doctor? We are a mystery. What does that mean, really? Its one of those things thats open to interpretation. Figure it out yourself.